Insulation above integral garage

[^woody^]

No you spend a few hours heating the walls up while the heat is gradually escaping the insulation - the wall will lose heat almost as fast as it takes to heat up. So you will need to have the heating on for many hours before the insulation actually starts performing, wasting energy. Heat is not radiated back into the room until the room temperature is lower than the wall you have just heated up (ie turned the heating off), and then at a very marginal rate to be effectively of no value in context of heating your home.

Whereas, if the insulation is right behind the plasterboard it is working almost instantly reflecting heat

 

[mikeey84]

Edit- stupid question

 

[cdbe]

The OP has twice stated that the floor void is filled with insulation but it appears to be inadequate. Fitting an additional layer of high performance insulation under the ceiling (the only practical place to fit it as no one is going to start ripping up floors or pulling down ceilings) is a perfectly good solution and will almost certainly make all the difference in this situation.

However, from a purely "cost" perspective, the OP would probably be better just increasing the heating output in the room - which was probably not calculated correctly when installed.